Network Working Group J. Palme
Request for Comments: 2557 Stockholm University/KTH
Obsoletes: 2110 A. Hopmann
Category: Standards Track Microsoft Corporation
N. Shelness
Lotus Development Corporation
March 1999
MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
HTML [RFC 1866] defines a powerful means of specifying multimedia
documents. These multimedia documents consist of a text/html root
resource (object) and other subsidiary resources (image, video clip,
applet, etc. objects) referenced by Uniform Resource Identifiers
(URIs) within the text/html root resource. When an HTML multimedia
document is retrieved by a browser, each of these component resources
is individually retrieved in real time from a location, and using a
protocol, specified by each URI.
In order to transfer a complete HTML multimedia document in a single
e-mail message, it is necessary to: a) aggregate a text/html root
resource and all of the subsidiary resources it references into a
single composite message structure, and b) define a means by which
URIs in the text/html root can reference subsidiary resources within
that composite message structure.
This document a) defines the use of a MIME multipart/related
structure to aggregate a text/html root resource and the subsidiary
resources it references, and b) specifies a MIME content-header
(Content-Location) that allow URIs in a multipart/related text/html
root body part to reference subsidiary resources in other body parts
of the same multipart/related structure.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
While initially designed to support e-mail transfer of complete
multi-resource HTML multimedia documents, these conventions can also
be employed to resources retrieved by other transfer protocols such
as HTTP and FTP to retrieve a complete multi-resource HTML multimedia
document in a single transfer or for storage and archiving of
complete HTML-documents.
Differences between this and a previous version of this standard,
which was published as RFC 2110, are summarized in chapter 12.
Table of Contents
1. Introduction ................................................. 3
2. Terminology ................................................. 4
2.1 Conformance requirement terminology ...................... 4
2.2 Other terminology ........................................ 4
3. Overview ..................................................... 6
4. The Content-Location MIME Content Header ..................... 6
4.1 MIME content headers ..................................... 6
4.2 The Content-Location Header .............................. 7
4.3 URIs of MHTML aggregates ................................. 8
4.4 Encoding and decoding of URIs in MIME header fields ...... 8
5. Base URIs for resolution of relative URIs .................... 9
6. Sending documents without linked objects ..................... 10
7. Use of the Content-Type "multipart/related" .................. 11
8. Usage of Links to Other Body Parts ........................... 13
8.1 General principle ........................................ 13
8.2 Resolution of URIs in text/html body parts ............... 13
8.3 Use of the Content-ID header and CID URLs ................ 14
9. Examples ..................................................... 14
9.1 Example of a HTML body without included linked objects ... 15
9.2 Example with an absolute URI to an embedded GIF picture .. 15
9.3 Example with relative URIs to embedded GIF pictures ...... 16
9.4 Example with a relative URI and no BASE available ........ 17
9.5 Example using CID URL and Content-ID header to an embedded
GIF picture .............................................. 18
9.6 Example showing permitted and forbidden references between
nested body parts ........................................ 19
10. Character encoding issues and end-of-line issues ............ 21
11. Security Considerations ..................................... 22
11.1 Security considerations not related to caching .......... 22
11.2 Security considerations related to caching .............. 23
12. Differences as compared to the previous version of this
proposed standard in RFC 2110 ............................... 24
13. Acknowledgments ............................................. 24
14. References .................................................. 25
15. Authors' Addresses .......................................... 27
16. Full Copyright Statement .................................... 28
Palme, et al. Standards Track [Page 2]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
1. Introduction
There are a number of document formats (Hypertext Markup Language
[HTML2], Extended Markup Language [XML], Portable Document format
[PDF] and Virtual Reality Markup Language [VRML]) that specify
documents consisting of a root resource and a number of distinct
subsidiary resources referenced by URIs within that root resource.
There is an obvious need to be able to send such multi-resource
documents in e-mail [SMTP], [RFC822] messages.
The standard defined in this document specifies how to aggregate such
multi-resource documents in MIME-formatted [MIME1 to MIME5] messages
for precisely this purpose.
While this specification was developed to satisfy the specific
aggregation requirements of multi-resource HTML documents, it may
also be applicable to other multi-resource document representations
linked by URIs. While this is the case, there is no requirement that
implementations claiming conformance to this standard be able to
handle any URI linked document representations other than those whose
root is HTML.
This aggregation into a single message of a root resource and the
subsidiary resources it references may also be applicable to
resources retrieved by other protocols such as HTTP or FTP, or to the
archiving of complete web pages as they appeared at a particular
point in time.
An informational RFC will be published as a supplement to this
standard. The informational RFC will discuss implementation methods
and some implementation problems. Implementers are strongly
recommended to read this informational RFC when developing
implementations of this standard. You can find it through URL
http://www.dsv.su.se/~jpalme/ietf/mhtml.html.
This standard specifies that body parts to be referenced can be
identified either by a Content-ID (containing a Message-ID value) or
by a Content-Location (containing an arbitrary URL). The reason why
this standard does not only recommend the use of Content-ID-s is that
it should be possible to forward existing web pages via e-mail
without having to rewrite the source text of the web pages. Such
rewriting has several disadvantages, one of them that security
checksums will probably be invalidated.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
2. Terminology
2.1 Conformance requirement terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [IETF-TERMS].
An implementation is not compliant if it fails to satisfy one or more
of the MUST requirements for the protocols it implements. An
implementation that satisfies all the MUST and all the SHOULD
requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST requirements but not all
the SHOULD requirements for its protocols is said to be
"conditionally compliant."
2.2 Other terminology
Most of the terms used in this document are defined in other RFCs.
Absolute URI, See Relative Uniform Resource Locators
AbsoluteURI [RELURL].
CID See Message/External Body Content-ID [MIDCID].
Content-Base This header was specified in RFC 2110, but has
been removed in this new version of the MHTML
standard.
Content-ID See Message/External Body Content-ID [MIDCID].
Content-Location MIME message or content part header with one
URI of the MIME message or content part body,
defined in section 4.2 below.
Content-Transfer- Conversion of a text into 7-bit octets as
Encoding specified in [MIME1] chapter 6.
CR See [RFC822].
CRLF See [RFC822].
Displayed text The text shown to the user reading a document
with a web browser. This may be different from
the HTML markup, see the definition of HTML
markup below.
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Header Field in a message or content heading
specifying the value of one attribute.
Heading Part of a message or content before the first
CRLFCRLF, containing formatted fields with
attributes of the message or content.
HTML See HTML 2 specification [HTML2].
HTML Aggregate HTML objects together with some or all objects,
objects to which the HTML object contains hyperlinks,
directly or indirectly.
HTML markup A file containing HTML encodings as specified
in [HTML] which may be different from the
displayed text which a person using a web
browser sees. For example, the HTML markup may
contain "<" where the displayed text
contains the character "
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
3. Overview
An aggregate document is a MIME-encoded message that contains a root
resource (object) as well as other resources linked to it via URIs.
These other resources may be required to display a multimedia
document based on the root resource (inline pictures, style sheets,
applets, etc.), or be the root resources of other multimedia
documents. It is important to keep in mind that aggregate documents
need to satisfy the differing needs of several audiences.
Mail sending agents might send aggregate documents as an encoding of
normal day-to-day electronic mail. Mail sending agents might also
send aggregate documents when a user wishes to mail a particular
document from the web to someone else. Finally mail sending agents
might send aggregate documents as automatic responders, providing
access to WWW resources for non-IP connected clients. Also with other
protocols such as HTTP or FTP, there may sometimes be a need to
retrieve aggregate documents. Receiving agents also have several
differing needs. Some receiving agents might be able to receive an
aggregate document and display it just as any other text content type
would be displayed. Others might have to pass this aggregate
document to a browsing program, and provisions need to be made to
make this possible.
Finally several other constraints on the problem arise. It is
important that it be possible for a document to be signed and for it
to be transmitted and displayed without breaking the message
integrity (MIC) checksum that is part of the signature.
4. The Content-Location MIME Content Header
4.1 MIME content headers
In order to resolve URI references to resources in other body parts,
one MIME content header is defined, Content-Location. This header can
occur in any message or content heading.
The syntax for this header is, using the syntax definition tools from
[ABNF]:
quoted-pair = ("\" text)
text = %d1-9 / ; Characters excluding CR and LF
%d11-12 /
%d14-127
WSP = SP / HTAB ; Whitespace characters
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FWS = ([*WSP CRLF] 1*WSP) ; Folding white-space
ctext = NO-WS-CTL / ; Non-white-space controls
%d33-39 / ; The rest of the US-ASCII
%d42-91 / ; characters not including "(",
%d93-127 ; ")", or "\"
comment = "(" *([FWS] (ctext / quoted-pair / comment))
[FWS] ")"
CFWS = *([FWS] comment) (([FWS] comment) / FWS)
content-location = "Content-Location:" [CFWS] URI [CFWS]
URI = absoluteURI | relativeURI
where URI is restricted to the syntax for URLs as defined in Uniform
Resource Locators [URL] until IETF specifies other kinds of URIs.
4.2 The Content-Location Header
A Content-Location header specifies an URI that labels the content of
a body part in whose heading it is placed. Its value CAN be an
absolute or a relative URI. Any URI or URL scheme may be used, but
use of non-standardized URI or URL schemes might entail some risk
that recipients cannot handle them correctly.
An URI in a Content-Location header need not refer to an resource
which is globally available for retrieval using this URI (after
resolution of relative URIs). However, URI-s in Content-Location
headers (if absolute, or resolvable to absolute URIs) SHOULD still be
globally unique.
A Content-Location header can thus be used to label a resource which
is not retrievable by some or all recipients of a message. For
example a Content-Location header may label an object which is only
retrievable using this URI in a restricted domain, such as within a
company-internal web space. A Content-Location header can even
contain a fictitious URI. Such an URI need not be globally unique.
A single Content-Location header field is allowed in any message or
content heading, in addition to a Content-ID header (as specified in
[MIME1]) and, in Message headings, a Message-ID (as specified in
[RFC822]). All of these constitute different, equally valid body part
labels, and any of them may be used to satisfy a reference to a body
part. Multiple Content-Location header fields in the same message
heading are not allowed.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
Example of a multipart/related structure containing body parts with
both Content-Location and Content-ID labels:
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"
--boundary-example
Content-Type: text/html; charset="US-ASCII"
... ... ... ...
... ... ... ...
--boundary-example
Content-Type: image/gif
Content-ID: <97116092511xyz@foo.bar.net>
Content-Location: fiction1/fiction2
--boundary-example
Content-Type: image/gif
Content-ID: <97116092811xyz@foo.bar.net>
Content-Location: fiction1/fiction3
--boundary-example--
4.3 URIs of MHTML aggregates
The URI of an MHTML aggregate is not the same as the URI of its root.
The URI of its root will directly retrieve only the root resource
itself, even if it may cause a web browser to separately retrieve
in-line linked resources. If a Content-Location header field is used
in the heading of a multipart/related, this Content-Location SHOULD
apply to the whole aggregate, not to its root part.
When an URI referring to an MHTML aggregate is used to retrieve this
aggregate, the set of resources retrieved can be different from the
set of resources retrieved using the Content-Locations of its parts.
For example, retrieving an MHTML aggregate may return an old version,
while retrieving the root URI and its in-line linked objects may
return a newer version.
4.4 Encoding and decoding of URIs in MIME header fields
4.4.1 Encoding of URIs containing inappropriate characters
Some documents may contain URIs with characters that are
inappropriate for an RFC 822 header, either because the URI itself
has an incorrect syntax according to [URL] or the URI syntax standard
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
has been changed to allow characters not previously allowed in MIME
headers. These URIs cannot be sent directly in a message header. If
such a URI occurs, all spaces and other illegal characters in it must
be encoded using one of the methods described in [MIME3] section 4.
This encoding MUST only be done in the header, not in the HTML text.
Receiving clients MUST decode the [MIME3] encoding in the heading
before comparing URIs in body text to URIs in Content-Location
headers.
The charset parameter value "US-ASCII" SHOULD be used if the URI
contains no octets outside of the 7-bit range. If such octets are
present, the correct charset parameter value (derived e.g. from
information about the HTML document the URI was found in) SHOULD be
used. If this cannot be safely established, the value "UNKNOWN-8BIT"
[RFC 1428] MUST be used.
Note, that for the matching of URIs in text/html body parts to URIs
in Content-Location headers, the value of the charset parameter is
irrelevant, but that it may be relevant for other purposes, and that
incorrect labeling MUST, therefore, be avoided. Warning: Irrelevance
of the charset parameter may not be true in the future, if different
character encodings of the same non-English filename are used in
HTML.
4.4.2 Folding of long URIs
Since MIME header fields have a limited length and long URIs can
result in Content-Location headers that exceed this length, Content-
Location headers may have to be folded.
Encoding as discussed in clause 4.4.1 MUST be done before such
folding. After that, the folding can be done, using the algorithm
defined in [URLBODY] section 3.1.
4.4.3 Unfolding and decoding of received URLs in MIME header fields
Upon receipt, folded MIME header fields should be unfolded, and then
any MIME encoding should be removed, to retrieve the original URI.
5. Base URIs for resolution of relative URIs
Relative URIs inside the contents of MIME body parts are resolved
relative to a base URI using the methods for resolving relative URIs
described in [RELURL]. In order to determine this base URI, the
first-applicable method in the following list applies.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
(a) There is a base specification inside the MIME body part
containing the relative URI which resolves relative URIs into
absolute URIs. For example, HTML provides the BASE element for
this purpose.
(b) There is a Content-Location header in the immediately surrounding
heading of the body part and it contains an absolute URI. This
URI can serve as a base in the same way as a requested URI can
serve as a base for relative URIs within a file retrieved via
HTTP [HTTP].
(c) If necessary, step (b) can be repeated recursively to find a
suitable Content-Location header in a surrounding multi-part or
message heading.
(d) If the MIME object is returned in a HTTP response, use the URI
used to initiate the request
(e) When the methods above do not yield an absolute URI, a base URL
of "thismessage:/" MUST be employed. This base URL has been
defined for the sole purpose of resolving relative references
within a multipart/related structure when no other base URI is
specified.
This is also described in other words in section 8.2 below.
6. Sending documents without linked objects
If a text/html resource (object) is sent without subsidiary
resources, to which it refers, it MAY be sent by itself. In this
case, embedding it in a multipart/related structure is not necessary.
Such a text/html resource may either contain no URIs, or URIs which
the recipient is expected to retrieve (if possible) via a URI
specified protocol. A text/html resource may also be sent with
unresolvable links in special cases, such as when two authors
exchange drafts of unfinished resources.
Inclusion of URIs referencing resources which the recipient has to
retrieve via an URI specified protocol may not work for some
recipients. This is because not all e-mail recipients have full
Internet connectivity, or because URIs which work for a sender will
not work for a recipient. This occurs, for example, when an URI
refers to a resource within a company-internal network that is not
accessible from outside the company.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
7. Use of the Content-Type "multipart/related"
If a message contains one or more MIME body parts containing URIs and
also contains as separate body parts, resources, to which these URIs
(as defined, for example, in HTML 2.0 [HTML2]) refer, then this whole
set of body parts (referring body parts and referred-to body parts)
SHOULD be sent within a multipart/related structure as defined in
[REL].
Even though headers can occur in a message that lacks an associated
multipart/related structure, this standard only covers their use for
resolution of URIs between body parts inside a multipart/related
structure. This standard does cover the case where a resource in a
nested multipart/related structure contains URIs that reference MIME
body parts in another multipart/related structure, in which it is
enclosed. This standard does not cover the case where a resource in a
multipart/related structure contains URIs that reference MIME body
parts in another parallel or nested multipart/related structure, or
in another MIME message, even if methods similar to those described
in this standard are used. Implementers who employ such URIs are
warned that receiving agents implementing this standard may not be
able to process such references.
When the start body part of a multipart/related structure is an
atomic object, such as a text/html resource, it SHOULD be employed as
the root resource of that multipart/related structure. When the start
body part of a multipart/related structure is a multipart/alternative
structure, and that structure contains at least one alternative body
part which is a suitable atomic object, such as a text/html resource,
then that body part SHOULD be employed as the root resource of the
aggregate document. Implementers are warned, however, that some
receiving agents treat multipart/alternative as if it had been
multipart/mixed (even though MIME [MIME1] requires support for
multipart/alternative).
[REL] specifies that a type parameter is mandatory in a "Content-
Type: multipart/related" header, and requires that it be employed to
specify the type of the multipart/related start object. Thus, the
type parameter value shall be "multipart/alternative", when the start
part is of "Content-type multipart/alternative", even if the actual
root resource is of type "text/html". In addition, if the
multipart/related start object is not the first body part in a
multipart/related structure, [REL] further requires that its
Content-ID MUST be specified as the value of a start parameter in the
"Content-Type: multipart/related" header.
Palme, et al. Standards Track [Page 11]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
When rendering a resource in a multipart/related structure, URI
references within that resource can be satisfied by body parts within
the same multipart/related structure (see section 8.2 below). This is
useful:
(a) For those recipients who only have email but not full Internet
access.
(b) For those recipients who for other reasons, such as firewalls or
the use of company-internal links, cannot retrieve URI referenced
resources via URI specified protocols.
Note, that this means that you can, via e-mail, send text/html
objects which includes URIs which the recipient cannot resolve
via HTTP or other connectivity-requiring URIs.
(c) To send a document whose content is preserved even if the
resources to which embedded URIs refer are later changed or
deleted.
(d) For resources which are not available for protocol based
retrieval.
(e) To speed up access.
When a sending MUA sends objects which were retrieved from the WWW,
it SHOULD maintain their WWW URIs. It SHOULD not transform these URIs
into some other URI form prior to transmitting them. This will allow
the receiving MUA to both verify MICs included with the message, as
well as verify the documents against their WWW counterpoints, if this
is appropriate.
In certain cases this will not work - for example, if a resource
contains URIs as parameters to objects and applets. In such a case,
it might be better to rewrite the document before sending it. This
problem is discussed in more detail in the informational RFC which
will be published as a supplement to this standard.
Within a multipart/related structure, each body part MUST have, if
assigned, a different Content-ID header value and a Content-Location
header field values which resolve to a different URI.
Two body parts in the same multipart/related structure can have the
same relative Content-Location header value, only if when resolved to
absolute URIs they become different.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
8. Usage of Links to Other Body Parts
8.1 General principle
A body part, such as a text/html body part, may contain URIs that
reference resources which are included as body parts in the same
message -- in detail, as body parts within the same multipart/related
structure. Often such URI linked resources are meant to be displayed
inline to the viewer of the referencing body part; for example,
objects referenced with the SRC attribute of the IMG element in HTML
2.0 [HTML2]. New elements and attributes with this property are
proposed in the ongoing development of HTML (examples: applet, frame,
profile, OBJECT, classid, codebase, data, SCRIPT). A sender might
also want to send a set of HTML documents which the reader can
traverse, and which are related with the attribute href of the A
element.
If a user retrieves and displays a web page formed from a text/html
resource, and the subsidiary resources it references, and merely
saves the text/html resource, that user may not at a later time be
able to retrieve and display the web page as it appeared when saved.
The format described in this standard can be used to archive and
retrieve all of the resources required to display the web page, as it
originally appeared at a certain moment of time, in one aggregate
file.
In order to send or store complete such messages, there is a need to
specify how a URI in one body part can reference a resource in
another body part.
8.2 Resolution of URIs in text/html body parts
The resolution of inline, retrieval and other kinds of URIs in
text/html body parts is performed in the following way:
(a) Unfold multiple line header values according to [URLBODY]. Do NOT
however translate character encodings of the kind described in
[URL]. Example: Do not transform "a%2eb/c%20d" into "a/b/c d".
(b) Remove all MIME encodings, such as content-transfer encoding and
header encodings as defined in MIME part 3 [MIME3] Do NOT however
translate character encodings of the kind described in [URL].
Example: Do not transform "a%2eb/c%20d" into "a/b/c d".
(c) Try to resolve all relative URIs in the HTML content and in
Content-Location headers using the procedure described in chapter
5 above. The result of this resolution can be an absolute URI, or
an absolute URI with the base "thismessage:/" as specified in
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
chapter 5.
(d) For each referencing URI in a text/html body part, compare the
value of the referencing URI after resolution as described in (a)
and (b), with the URI derived from Content-ID and Content-
Location headers for other body parts within the same or a
surrounding Multipart/related structure. If the strings are
identical, octet by octet, then the referencing URI references
that body part. This comparison will only succeed if the two URIs
are identical. This means that if one of the two URIs to be
compared was a fictitious absolute URI with the base
"thismessage:/", the other must also be such a fictitious
absolute URI, and not resolvable to a real absolute URI.
(e) If (d) fails, try to retrieve the URI referenced resource
hyperlink through ordinary Internet lookup. Resolution of URIs of
the URL-types "mid" or "cid" to other content-parts, outside the
same multipart/related structure, or in other separately sent
messages, is not covered by this standard, and is thus neither
encouraged nor forbidden.
8.3 Use of the Content-ID header and CID URLs
When URIs employing a CID (Content-ID) scheme as defined in [URL] and
[MIDCID] are used to reference other body parts in an MHTML
multipart/related structure, they MUST only be matched against
Content-ID header values, and not against Content-Location header
with CID: values. Thus, even though the following two headers are
identical in meaning, only the Content-ID value will be matched, and
the Content-Location value will be ignored.
Content-ID:
Content-Location: CID: foo@bar.net
Note: Content-IDs MUST be globally unique [MIME1]. It is thus not
permitted to make them unique only within a message or within a
single multipart/related structure.
9. Examples
Warning: The examples are provided for illustrative purposes only. If
there is a contradiction between the explanatory text and the
examples in this standard, then the explanatory text is normative.
Notation: The examples contain indentation to show the structure, the
real objects should not be indented in this way.
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
9.1 Example of a HTML body without included linked objects
The first example is the simplest form of an HTML email message. This
message does not contain an aggregate HTML object, but simply a
message with a single HTML body part. This body part contains a URI
but the messages does not contain the resource referenced by that
URI. To retrieve the resource referenced by the URI the receiving
client would need either IP access to the Internet, or an electronic
mail web gateway.
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit

Acute accent

9.2 Example with an absolute URI to an embedded GIF picture
The second example is an HTML message which includes a single image,
referenced using the Content-Location mechanism.
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"; start=""
--boundary-example
Content-Type: text/html;charset="US-ASCII"
Content-ID:
... text of the HTML document, which might contain a URI
referencing a resource in another body part, for example
through a statement such as: